Method for growing single crystals of iib-vib compounds



Dec. 19-70 I TADASHI YAMANAKA ETAL ,5 1, METHOD FOR GROWING SINGLE CRYSTALS 0F Il -V1 COMPOUNDS Original Filed April 25, 1966 F I g I b 2 i 7 W W Condensing 20m. evaporating zone Condensing zone 4 Fig. 2b

INVENTORS VGPOFG ting TADASHl YAMANAKA AKIRA MATSUBARA ZONE BY ATTORNEY United States Patent 3,551,117 METHOD FOR GROWING SINGLE CRYSTALS OF H -VI COMPOUNDS Tadashi Yamanaka and Akira Matsubara, Osaka-shi, Japan, assignors to Matsushita Electric Industrial (30., Ltd., Osaka, Japan Continuation of application Ser. No. 545,120, Apr. 25, 1966. This application Dec. 8, 1969, Ser. No. 878,981 Int. Cl. C01g 11/00,'9/00; C01b 19/20 US. Cl. 23-315 8 Claims ABSTRACT OF THE DISCLOSURE A vapour deposition method for growing a single crystal of selenides and tellurides of Cd and Zn, which comprises placing starting material which is a combination of metals selected from Cd, Zn, Se and Te and compounds selected from CdSe, CdTe, ZnSe and ZnTe at the closed end of a crystal growing tube, which has a closed bottom at one end and a pin-hole at the other end, and heating it at a sublimation temperature of the starting material. The material is heated in a furnace in an inert gas, and in which the temperature difference between the closed end and the pinhole end is 25 C. or more, and a single crystal is produced in a block form in the vicinity of the pin-hole.

This application is a continuation of Ser. No. 545,120, filed Apr. 25, 1966, and now abandoned.

This invention relates to a method for growing single crystals of II -VI compounds and their modifications and more particularly to a method for growing single crystals of II VI compounds and their modifications by employing a vapour deposition method. The classifications of the compounds used in this specification follow the Mendeleff table as printed in Sidgwick, The Chemical Elements and Their Compounds, Oxford, The Clarendon Press, 1950, page xxviii.

In the periodic table 11,; elements such as Zn and Cd form an interesting compound with VI elements such as S, Se, Te. The produced compounds such as ZnS, ZnSe, ZnTe, CdS, CdSe and CdTe are generally named as II VT compounds. In convenience, the II -VI compounds referred hereafter will include CdSe, CdTe, ZnSe, ZnTe and also their modifications containing three elements from the two groups in various combinations such as Cd Zn Se, Cd Zu Te, CdSe Te and ZnSe Te The single crystals of these II -VI compounds have a high potential for use in elements for p-n junction devices such as photovoltaic cells, electroluminescence applications and various diodes. Single crystals of the II VI compounds have been prepared heretofore by a melting method or a vapour deposition or sublimation methods. Since the 11 V1,; compounds have usually a high vapour pressure at high temperatures, the melting method is required to employ a high pressure equipment for producing single crystals thereof. Furthermore, the melting method can not avoid the contamination of single crystals from the crucible material during melting. The vapour deposition and sublimation methods are suitable for growing such a single crystal having a high vapour pressure but have the drawback that single crystals of large size are ditficult to be produced. The conventional vapour deposition method is performed by sealing starting materials in an evacuated tube and heating it at a temperature gradient which is ad justed so as to produce a crystal depending on the ma terial. According to another procedure of the vapour depo sition, the starting materials are heated in a temperature gradient furnace while flowing an inert gas such as, for example, nitrogen, helium and argon. Single crystals grow at the lower temperature region of the furnace during Patented Dec. 29, 1970 deposition of the vapour formed from the starting materials at the higher temperature region. These methods usually produce a needle-like crystal or a plate-like crystal but it is ditficult to produce a large crystal in a block form. Another, disadvantage of these methods are a requirement for strict control of the temperature schedule during operation. In addition, a sealing method is indispensable to prevent an explosion caused by an excess amount of the starting materials.

An object of this invention is to provide a vapour deposition method for growing a single crystal of II VI compounds in a large block form.

Another object of this invention is to provide a vapour deposition method for growing a single crystal of II -VI compounds, which comprises no complicated temperature schedule during operation.

Further object of this invention is to provide a vapour deposition method for growing a single crystal of II VI compounds, which suffers no dangerous explosion during operation.

These and other objects will be readily apparent to those skilled in the art from an examination of the following description and accompanying drawings wherein:

FIG. 1a is a cross sectional view of a crystal growing tube having a pin-hole at the center thereof in accordance with this invention.

FIG. 1b is a cross sectional view of a crystal growing tube having a pin-hole at a position deviated from the center thereof in accordance with this invention.

FIG. 2a illustrates a position of the crystal growing tube of FIG. 1b in horizontal furnace.

FIG. 2b illustrates a position of the crystal growing tube of FIG. 1b in a vertical furnace.

According to the present invention, there is provided a process for the production of single crystals of II VI compounds by subjecting such compounds to a variable heat zone, whereby the compounds are first exposed to a temperature zone heated to a temperature to cause evaporation of the compounds, and then exposed to a lower temperature zone wherein the compounds condense to form a single crystal, the heating and cooling being carried out in a tube having one closed bottom end and one open top end provided with a pin-hole which is subjected to the condensing temperature, the crystals being formed adjacent to such pin-hole. During the heating, the compounds are in the presence of an inert gas.

Referring to FIG. 1a, a whole drawing represents a crystal growing tube which contains starting materials and is to be heated in a furnace for preparing a single crystal from the starting materials. According to this invention it has been discovered that a single crystal 4 of II VI compounds be prepared in a form having a pipe-shaped hollow channel 5 by employing such a crystal growing tube 1 that has a small pin-hole 2 at one end and a closed bottom 6 at the other end. The starting materials 3 are placed inside said closed bottom. The crystal growing tube containing the starting materials is heated in a flowing inert gas such as nitrogen, helium or argon in a furnace 7 having a temperature gradient which is adjusted so as to evaporate the starting materials at the closed bottom and deposit the produced vapour in the vicinity of said pinhole 2.

The size of said pin-hole has a great effect on the growth of the single crystal. Preferably, the size of said pin-hole is 0.1 mm. to 0.8 mm. in diameter. Said pin-hole in the preferable size makes it possible to deposit the produced vapour in the vicinity of said pin-hole 2 even when the temperature of said pin-hole is close to a sublimation temperature at which the starting materials are maintained so as to produce a suitable amount of vapour. For example, a single crystal of ZnTe grows predominantly in the vicinity of said pin-hole at 1250 C. when the starting materials are heated at 1275 C. in the crystal growing tube having a pin-hole of 0.5 mm. in diameter. However, no single crystal grows in the vicinity of the pin-hole when the size is larger than 2 mm. in diameter even when the pin-hole is maintained at 1000 C. and the starting materials are heated at 1275 C. The produced vapour escapes easily through the large pin-hole to the outside even when the temperature gradient is sufiiciently large as mentioned above. Therefore, a single crystal of II -VI compounds can be prepared efliciently, even at a small temperature gradient, by employing the crystal growing tube having a pin-hole in a diameter of 0.1 to 0.8 mm.

Said pin-hole is not necessary to have a specified shape. Any kind of shape can be used, for example a circle or a square. Preferable opening area of said pin-hole is 0.01 rnm. to 1 mm.

Said crystal growing tube can be made from any kind of refractory material inert to both the starting materials and the produced single crystals at high temperatures. Operable materials are quartz, porcelain and platinum. Quartz is preferable because of the easiness of fabrication to a crystal growing tube. One can use a crystal growing tube having any shape in cross section, any diameter and any length thereof.

The crystal growing tube according to this invention can prepare a large single crystal of II VI compounds such as CdSe, CdTe, ZnSe and ZnTe and their three element combinations such as Cd Zn Se, Cd Zn Te, CdSe Te and ZnSe Te It is possible to use any form of starting materials, for example, a mixture of each component such as a mixture of Zn, Te and Se or a compound such as ZnSe and ZnTe.

For example, quartz is fabricated into said crystal growing tube having said pin-hole in a conventional glass i blower technique. Then the starting material is inserted and the tube with the starting materials is evacuated and flushed with an inert gas such as nitrogen, helium or argon a few times. The tube containing the starting materials is heated under flowing inert gas in a furnace 7 having a given temperature gradient. The highest temperature of the temperature gradient depends upon the vapour pressure of starting materials. The preferable sublimation temperature is 900 C. to 1300 C. with II3-VIB compounds and their modifications. The temperature of said pin-hole is necessary to be lower at least by 25 C. than that of the starting materials, i.e., the sublimation temperature. Preferable temperature gradient is such that the temperature difference between said pin-hole and starting materials is greater than 75 C. The crystal growing tube containing the starting materials is heated in a flowing inert gas at such a temperature gradient for 30 to 70 hours and then furnace-cooled to room temperature. A single crystal grows predominantly in the vicinity of said pin-hole leaving a pipe-shaped hollow channel through it as shown in FIG. 1a.

It has been discovered, according to the present invention, that when said pin-hole 2 is made at a position deviated from the longitudinal axis of said tube, the crystals 4 have said pipe-shaped hollow 5 in a position deviated from the axial center line as shown in FIG. 1b. Therefore, the deviated pin-hole can make the size of the available single crystal larger.

It is preferable to place said crystal growing tube having the deviated pin-hole in such a way that said tube inclines against the wall of the furnace tube 7 at an angle of 10 to 60 and the deviated pin-hole is kept upwardly with respect to said axial center line as shown in FIG. 2.

Said crystal growing tube can be fed with a large amount of starting materials without any explosion, but it becomes very difficult to hold the temperature of the surface level of a large amount of the starting materials at a suitable sublimation temperature. This difiiculty is solved by moving said tube mechanically, that is, shifting the position of said tube or a furnace in order to produce 4 a preferable vapour pressure of the starting materials.

The following examples set forth specified embodiments of this invention and should not be construed as limitative.

EXAMPLE 1 Zinc metal of 6.54 gr. and tellurium metal of 16.59 gr. were placed within a tube of 11 mm. inside diameter and mm. in length which was closed at one end and had a pin-hole of 0.5 mm. in diameter at the other end. This tube was evacuated and flushed with nitrogen gas three times, finally left with 1 atm. of nitrogen gas and then placed in a furnace tube of 50 mm. inside diameter and 1000 mm. in length. Nitrogen gas was flowed through the furnace tube during the run at a rate of cc./min. The charged starting material was heated slowly at initial periods and finally heated at 1250 C. for 63 hours. The temperature of the starting material was about 1250 C. and the temperature near the pin-hole was about 900 C. Single crystals of ZnTe grew on the wall near the pinhole. The product was a block of 28 mm. in length and 11 mm. in diameter which had a hollow channel through it as shown in FIG. 1a. A single crystal of ZnTe having dimensions of 10 x 8 x 5 mm. was cleaved from the said block. Two parallel cleaved faces were identified as (110) planes by usual X-ray method.

The ZnTe crystals prepared by this method can be used for electroluminescence diodes. The ZnTe crystals were sliced into pellets in a size of 1.8 x 1.8 x 0.8 mm. so as to make both faces a (110) plane. Diodes were prepared by applying InAg alloy onto a (110) face of ZnTe crystals in hydrogen atmosphere at temperature of 750 C. When the diodes were biased reversely at temperature of 196 C., the light having a peak at a wavelength 5400 A. was emitted.

EXAMPLE 2 Zinc metal of 5.00 gr. and tellurium metal of 10.74 gr. were placed within a quartz tube of 10 mm. inside diameter and 100 mm. in length which was closed at one end and had a pin-hole of 0.3 mm. in diameter at the other end. As shown in FIG. lb said pin-hole was located in a position deviated 3 mm. from the center axial line of the tube. This tube was evacuated and placed in a furnace tube in the same manner as that described above. The starting material in said tube was heated slowly at initial periods and finally maintained at 1300 C. for 50 hours. The temperature of the starting material was about 1300 C. and the temperature near the pin-hole was about 1200 C. The product was a block of 28 mm. in length and 9 mm. in diameter through which a hollow channel existed upwards of the center line of the block. A single crystal of 8 x 5 X 3 mm. was cut out from the said block.

. EXAMPLE 3 Zinc metal of 5.00 gr. and tellurium metal of 14.64 gr. were placed within a tube of 9 mm. inside diameter and 100 mm. in length which was closed at the bottom and had a pin-hole of 0.5 mm. in diameter at the top. This tube was evacuated, flushed and left with 1 atm. of nitrogen gas and then placed in a vertical furnace tube of 42 mm. inside diameter and 1000 mm. in length. Nitrogen gas was flowed through the furnace tube during the run at a rate of 100 cc./min. The temperature of the starting material was maintained at 1250 C. for 70 hours. The maximum temperature point of the furnace was adjusted to be positioned in the neighborhood of the top of the charged starting material and the temperature of said pinhole was kept 950 C. The tube was mechanically moved upwards at a rate of 1.5 mm./hr. When the point of the maximum temperature shifted to the bottom of the tube, the sublimation run terminated. The product was a block of 23 mm. in length and 9 mm. in diameter which had a hollow. A single crystal of ZnTe of 8 x 5 x 3 mm. was cut out from the block.

EXAMPLE 4 CdSe powder of 5 gr. was placed within a tube which was closed at one end and had a pin-hole of 0.3 min. in diameter at the other end as shown in FIG. 1a. This tube was evacuated and placed in a furnace tube in the same manner as given in Example 1. The tube was heated at 1200 C. for 54 hours. The temperature of said pin-hole was 1120 C. during sublimation run. A CdSe single crystal of 3 x 2 x 2 mm. was obtained.

EXAMPLE 5 Zinc metal of 1.31 gr., tellurium metal of 1.28 gr. and selenium metal of 0.79 gr. were placed within a quartz tube which was closed at one end and had a pin-hole of 0.5 mm. in diameter at the other end as shown in FIG. 1a. This tube was evacuated and placed in a furnace tube in the same manner as given in Example 1. The starting material in said tube was heated at 1300 C. for 4 hours. The temperature of said pin-hole was about 1200 C. A single crystal of ZnTe Se having dimensions of 1.5 x 1 x 1 mm. was obtained.

We claim:

1. A vapour deposition method for growing a single crystal of II VI compounds selected from the binary compounds CdSe, CdTe, ZnSe and ZnTe which comprises; placing starting material selected from one member of the group consisting of Cd and Zn and one member of the group consisting of Se and Te in an unsealed, refractory, inert crystal growing tube having a closed bottom at one end and a pin-hole which is smaller than 2 mm. in diameter at the other end, said starting material being positioned at said closed bottom; heating said crystal growing tube in an inert gas in a temperature gradient furnace with said gradient arranged with respect to said tube so that said starting material is heated at a sublimation temperature of said starting material between 900 C. and 1300 C. and the temperature of said pin-hole is lower by at least 75 C. than the temperature of said starting material whereby a said single crystal in a block form is formed in the vicinity of said pin-hole.

2. A vapour deposition method for growing a single crystal of II -VI compounds as defined in claim 1, wherein said pin-hole has a diameter of 0.1 to 0.8 mm.

3. A. vapour deposition method for growing a single crystal of Il -VI compounds as defined in claim 1, wherein said pin-hole has an opening area of 0.01 ,to 1 mm.

4. A vapour deposition method for growing a single crystal of II -VI compounds as defined in claim 1, wherein said pin-hole is in a position deviated from an axial center line of said crystal growing tube.

5. A vapour deposition method for growing a single crystal of II -VI compounds as defined in claim 1, wherein said pin-hole is in a position deviated from an axial center line of said crystal growing tube and said crystal growing tube inclines against the wall of a furnace having temperature gradient at an angle of 10 to with said deviated pin-hole upwardly with respect to said axial center line.

6. A vapour deposition method for growing a single crystal of zinc telluride which comprises: placing starting material consisting of Zn and Te in an unsealed, refractory, inert crystal growing tube having a closed bottom at one end and a pin-hole of 0.1 to 0.8 mm. diameter at the other end, said starting material being positioned at said closed bottom; heating said crystal growing tube in a flowing inert gas in a temperature gradient furnace with said gradient arranged with respect to said tube so that the temperature of said starting material is higher than 1200 C. and the temperature of said pin-hole is lower by at least C. than that of said starting material; whereby a single crystal of zinc telluride in a block form is formed in the vicinity of said pin-hole.

7. A vapour deposition method for growing a single crystal of a Zn, Te, Se compound which comprises: placing starting material consisting of Zn, Te and Se in an unsealed refractory, inert crystal growing tube having a closed bottom at one end and a pin-hole of 0.1 to 10.8 mm. diameter at the other end, said starting material being positioned at said closed bottom; heating said crystal growing tube in a flowing inert gas in a temperature gradient furnace with said gradient arranged with respect to said tube so that the temperature of said starting material is higher than 1200 C. and the temperature of said pin-hole is lower by at least 75 C. than that of said starting material; whereby a single crystal of a Zn, Te, Se compound in a block form is formed in the vicinity of said pin-hole.

8. A method as set forth in claim 7 wherein said starting material is heated to about 1300 C. and said pinhole is heated to about 1200 C.

References Cited UNITED STATES PATENTS 2,862,787 12/1958 Seguin et al 1 23-50X 2,995,613 8/1961 Wernick 235OX 3,297,403 1/1967 Haacke 23--20X 3,342,546 9/1967 Reisman et al 23-50 HERBERT T. CARTER, Primary Examiner US. Cl. X.R. 

